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Re: thermostats & fan switchers...running a little hot!
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 31 Jul 2008 09:15:29 Share Post by Email
In Reply to: thermostats & fan switchers...running a little hot!, Steven W, Wed, 30 Jul 2008 19:59:03
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To lead off, a cooler fan thermoswitch is a fine idea. And a cooler thermostat is OK, also. Best done as a package. Downside? Not big. There is a theoretical slight reduction in gas mileage running the car with a cooler thermostat, but I doubt you'd see it in real life. Same with the electricity to use the fan. Theoretical, yes, reality, no way.

If you live where it gets REALLY cold - regularly below 0 F, then you might notice less heat with a cooler thermostat. Anywhere else, you won't notice it. The radiator fan switch doesn't matter for heat.

Before you get too worried, it's important to understand the cooling system and what the gauge tells you. There are two cooling loops - engine and radiator. The engine loop has the engine and the temperature gauge. So the gauge reads engine temperature. The radiator loop is connected to the engine by the thermostat. When the engine gets warm enough, it sends coolant to the radiator to cool off. Or that's the theory, anyway.

The fan switch is on the radiator. It turns on and off depending on how hot the radiator is, not how hot the engine is. Related but not the same. The idea of the radiator fan is to replace the airflow that is missing when the car isn't moving. If the car is moving more than about 25 mph, that airflow is way more than what the fan can provide.

So if you see the temp gauge going up while the car is moving at reasonable speeds, suspect a sticky thermostat or a clogged radiator. No amount of fan is going to help there.

However, say you're sitting at a light or stuck in traffic. The engine gets warmer and warmer. The thermostat is open, and that hot coolant moves to the radiator. While driving, the airflow over the radiator will cool it off, and that cooler coolant moves to the engine. But stuck in traffic, you don't get that cooling airflow. So the radiator coolant gets hotter and hotter, as does the coolant in the engine. Finally the radiator fan switch turns on the fans - the airflow cools off the coolant, and the engine temp drops.

A cooler radiator fan thermoswitch will turn that fan on sooner. If the needle going high happens only when not moving, it will keep things a little cooler.

However, this is probably more of a psychological issue than mechanical. The needle gets *near* the red, so we worry. But the engine is designed to work there just fine. Yes, we'd all like some margin there, but that isn't real. If something goes wrong in the cooling system, having 10 degrees C of margin won't matter - the engine temp will blow through that in a few seconds on the way to overheating.

That said, assuming the cooling system is working properly, putting in a cooler fan switch is just fine. It will run a little more often, and the needle won't go quite as high before the fan kicks in.

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